U.S. Pat. No. 6,157,035 describes how a fast detector in a standard CMOS technology can be constructed. A shadow mask gives a spatial modulation to the incident, and consequently of the absorbed light in the semiconductor substrate. By measuring the magnitude of the spatial frequency component in the minority carrier distribution with a spatial frequency corresponding to that of the shadow mask, a fast detector is provided. Speed is hereby enhanced but the traded-off is sensitivity.
WO 98/10255 and WO 99/60629 show methods and corresponding devices for determining the phase and/or amplitude of incident modulated light. These pixel structures are called photonic mixers. These structures are useful for making range-finding cameras giving depth or 3D information as is explained in these patent applications. By applying a modulation voltage over two photo-gates, the generated minority carriers in the substrate arrive preferentially at one of two detector accumulation zones. In WO 98/10255, these accumulation zones are potential wells, created by a voltage on an adjacent accumulation gate. In WO 99/60629, these accumulation zones are pn-junctions. There are however two major deficiencies with these photonic mixers. Firstly, for light that is absorbed more deeply in the substrate, the generated minority carriers will have to diffuse towards the surface before the modulation photo-gates can redirect these carriers towards the destination accumulation zones. This diffusion process over a 5 to 20 μm distance limits the speed with which demodulation can be achieved. Second, the percentage of detected minority carriers that reach effectively the target accumulation zone is far from optimal.